This invention relates to gas turbine engine rotor blades with leading and trailing edges having localized compressive residual stresses imparted by laser shock peening and, more particularly, to techniques for countering laser shock peening induced twist of the airfoils of these blades.
Laser shock peening (LSP) or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of an article. Laser shock peening typically uses one or more radiation pulses from high and low power pulsed lasers to produce an intense shockwave at the surface of an article similar to methods disclosed in U.S. Pat. No. 3,850,698 entitled “Altering Material Properties”; U.S. Pat. No. 4,401,477 entitled “Laser Shock Processing”; and U.S. Pat. No. 5,131,957 entitled “Material Properties”. Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of the surface. The portion of the surface may have an ablative coating or be bare, meaning having no ablative coating. An explosive force is produced at the impingement point of the laser beam by an instantaneous ablation or vaporization of a thin layer of the material surface or of a coating (such as tape or paint) on the surface which forms a plasma.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. No. 5,756,965 entitled “On The Fly Laser Shock Peening”; U.S. Pat. No. 5,591,009 entitled “Laser shock peened gas turbine engine fan blade edges”; U.S. Pat. No. 5,531,570 entitled “Distortion control for laser shock peened gas turbine engine compressor blade edges”; U.S. Pat. No. 5,492,447 entitled “Laser shock peened rotor components for turbomachinery”; U.S. Pat. No. 5,674,329 entitled “Adhesive tape covered laser shock peening”; and U.S. Pat. No. 5,674,328 entitled “Dry tape covered laser shock peening”, all of which are assigned to the present Assignee.
High energy laser beams, from about 20 to about 50 Joules, or low energy laser beams, from about 3 to about 10 Joules, have been used and other levels are contemplated. See, for example, U.S. Pat. No. 5,674,329 (Mannava et al.) issued Oct. 7, 1997 (LSP process using high energy lasers) and U.S. Pat. No. 5,932,120 (Mannava et al.) issued Aug. 3, 1999 (LSP process using low energy lasers). Low energy laser beams can be produced using different laser materials such as neodymium doped yttrium aluminum garnet (Nd YAG), Nd:YLF, and others. Laser shock peening processes typically employ a curtain of water or other confinement liquid medium flowed over the article or some other method to provide a plasma confining medium. This medium enables the plasma to rapidly achieve shockwave pressures that produce the plastic deformation and associated residual stress patterns that constitute the LSP effect. The curtain of water provides a confining medium, to confine and redirect the process generated shockwaves into the bulk of the material of a component being LSP'D, to create the beneficial compressive residual stresses.
The LSP process generates deep compressive stresses in the article resulting in improved fatigue strength under foreign object damage (FOD) conditions. LSP improves material properties such as high cycle fatigue, low cycle fatigue, corrosion & erosion resistance. Laser shock peening of leading and/or trailing edges of fan, compressor, and turbine blade airfoils typically includes coating the portion of the edges with an ablative material, such as paint or tape, to provide the material for the plasma. The laser shock peening may be performed on bare or uncoated surfaces. There are associated distortions of thin compressor and turbine blades that alter the profile of the blade and, therefore, may cause substantial aerodynamic problems. These associated distortions include an alteration of twist of the airfoil of the blades. It is highly desirable to reduce or eliminate this laser shock peened induced twist of the airfoil.